Attention makes it possible to filter stimulus attributes that are not task-related. To examine attentional filtering of information for short-term visual memory we used stimuli generated by superimposing horizontal and vertical Gabor patches. On each trial, subjects saw and tried to remember a pair of sequentially-presented study items, which were followed by a probe. Subjects judged whether the probe's spatial frequency matched that of a study item. For half the subjects, the vertical component only was task-relevant; for other subjects, the horizontal component was task-relevant. All stimuli were scaled for individual subjects' discrimination thresholds.

For the task-relevant orientation, recognition judgments and latencies varied systematically with the summed similarity of the probe to each study item, which is consistent with our lab's previous work with visual and auditory stimuli. But the task-irrelevant orientation exerted an effect of its own. When probe and study item frequencies matched not only the task-relevant orientation, but also the task-irrelevant one, recognition performance was boosted by about 20%, and responses were speeded.

To illuminate the mechanism by which the task-irrelevant attribute exerts its influence, we used NEMO, a noisy-exemplar model of recognition (Sekuler & Kahana, 2002, 2007). Fitting data to alternative forms of NEMO dissociated the influences of (i) summed probe-item similarity and (ii) inter-item homogeneity, another variable known to influence recognition. Task-irrelevant information contributed only to summed similarity, suggesting that attentional resources are needed for the computation of inter-item homogeneity. This finding is consistent with previous speculation that inter-item homogeneity affects the decision criterion rather than memory strength.